1. Field of the Invention
The present invention relates to laminated inductors, and more particularly, to a laminated inductor included in a choke coil and other suitable devices.
2. Description of the Related Art
Conventionally, a laminated inductor is produced by laminating a plurality of magnetic layers with a plurality of coil conductors to form a composite body and then baking the composite body, and forming a coil in the composite body by electrically connecting the plurality of coil conductors. In addition, the composite is provided at a surface thereof with input/output external electrodes electrically connected to ends of the coil.
To decrease direct current resistance in a laminated inductor, silver (Ag) has been used as the coil conductor. Accordingly, a material used for magnetic layers must be sinterable at a temperature up to the melting point of the Ag (approximately 960xc2x0 C.). For this reason, conventionally, to obtain a material used as the magnetic layer material, Fe2O3, NiO, ZnO, and CuO are mixed together in a ball mill or other suitable device and then calcined. Further, the calcined mixture is pulverized and mixed with a binder, whereby a slurry used as the magnetic layer material is obtained. Alternatively, a sheet made from the slurry by a doctor blade method is used for the magnetic layer. To decrease the calcination and firing temperature, Bi2O3 is added to the Nixe2x80x94Znxe2x80x94Cu system ferrite magnetic material described above. However, since the particle diameter of conventional Bi2O3 is relatively large (a small specific surface area of 5 to 6 m2/g), it is difficult to handle the Bi2O3 because the dispersibility thereof is poor when mixed with Fe2O3, NiO, ZnO, and CuO, and variations tend to occur regarding degree of calcination synthetic reaction and magnetic characteristics after firing. When the amount of Bi2O3 added is relatively small, particle diameters of the magnetic material after baking are generally not uniform, and when the amount added thereof is increased to make the particle diameters more uniform, magnetic characteristics generally decrease (in particular, inductance is reduced).
In addition, when external electrodes are plated, the plating solution penetrates into the composite and reacts with the coil conductors. As a result, an increase in direct current resistance and degradation of characteristics relating to breakdown voltage between electrodes often occur. Furthermore, a phenomenon (hereinafter referred to as xe2x80x9cplating growthxe2x80x9d) may occur in which a plating layer grows abnormally and extends from edges of the external electrodes onto the surfaces of the composite. As a result, short-circuiting is caused between the external electrodes of a compact laminated inductor and characteristics relating to breakdown voltage between the external electrodes is reduced. Consequently, before performing a plating treatment, countermeasures, such as a glass coating layer provided between the external electrodes, are often necessary.
To overcome the above-described problems, preferred embodiments of the present invention provide a laminated inductor, in which a synthetic reaction in a magnetic layer is substantially improved during calcination and firing, and the problems encountered by the prior art laminated inductors are eliminated.
The laminated inductor according to a preferred embodiment of the present invention preferably includes a laminated composite body including a plurality of magnetic layers and a plurality of coil conductors, the plurality of coil conductors being electrically connected to define a coil, and external electrodes provided on surfaces of the composite body and connected to ends of the coil, wherein the magnetic layers are made of a Nixe2x80x94Znxe2x80x94Cu system ferrite magnetic material including a powder including Fe2O3, NiO, ZnO, and CuO, to which Bi2O3 having a specific surface area of about 10 m2/g to about 20 m2/g is added by approximately 0.1 to less than about 0.5 percent by weight. The composition of the powder including Fe2O3, NiO, ZnO, and CuO as major components is preferably about 45 to about 50 mole percent of Fe2O3, about 5 to about 50 mole percent of NiO, about 0.5 to about 30 mole percent of ZnO, and about 4 to about 16 mole percent of CuO.
Since the Bi2O3 having a specific surface area of about 10 m2/g to 20 m2/g is included in the Nixe2x80x94Znxe2x80x94Cu system ferrite magnetic material, even though the amount of the Bi2O3 is less than that of the conventional Bi2O3 having a specific surface area of about 5 m2/g to about 6 m2/g, plating growth from the external electrodes is minimized and eliminated while the magnetic characteristics are greatly improved.
Additionally, when Mn2O3 is further added by about 0.05 to about 0.3 percent by weight to the powder primarily including Fe2O3, NiO, ZnO, and CuO, resistivity of the magnetic layer is greatly increased without impairing sintering density, water absorption rate, and initial permeability.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.